Removes startedRegistration property added in #166

[aaronschachter]

What's this PR do?

This pull request removes the convenience startedRegistration property added to a RockTheVoteRecord in #166 -- because older failed jobs in the queue don't have it set.

This results in the error:

 Undefined property: Chompy\RockTheVoteRecord::$startedRegistration   

when we retry these failed jobs.

Instead, we can use the record's getPostDetails helper to inspect the record's Started registration datetime.

How should this be reviewed?

👀

Any background context you want to provide?

🌵

Relevant tickets

References Pivotal #172805635.

Checklist

• [ ] This PR has been added to the relevant Pivotal card.
• [ ] Documentation added for new features/changed endpoints.
• [ ] Added appropriate feature/unit tests.